There seems to be an immediate understanding between Scout and Boo. Why?

I love this scene in the book. Boo comes out of the shadows onto Scout's porch. There is none of her earlier childish statements or behaviors. She is not scared and, unlike many adults, does not look upon Boo as an oddity. She simple says "Hey Boo". This gentle greeting shows how sensitive Scout has become. It is friendly recognition but also has the subtext that says so much. The subtext underneath her tone speaks to Scout's feelings that she knew it all along. She may have not comprehended Boo before but somewhere under the childish stories and games, Scout was sensitive enough to realize that Boo was not a monster.Boo understands that Scout isn't so childish anymore and that maybe he has a friend.